During operation, gas turbine engines, whether used for flight or stationary power generation, develop extremely high temperature and high velocity gases in a combustor portion of the engine. These gases are ducted on blades of a turbine rotor to cause rotation of the rotor and are redirected by the stator vanes onto additional rotor blades to produce more work. Because of the high heat of the gases, it is desirable to cool the blades and vanes to prevent damage and, to extend the useful life of, these engine components. It is known in the art that a turbine component such as that shown in FIG. 9 can be cooled by film cooling that is provided by a plurality of cooling holes.
A commonly used method of cooling a turbine component 20 is to duct cooling air through internal passages and then vent the cooling air through a plurality of cooling holes 22. This air cools internal surfaces of the component by convection and cools the components outer surfaces by film cooling. The cooling holes 22 are typically formed along a line substantially parallel to, and a selected distance from, a leading edge 24 of the component to provide a film of cooling air over a surface of the component when the cooling holes discharge air during engine operation. Other rows or arrays of cooling holes or vents may be formed in the blade and vane components of a rotor or stator of a turbine depending upon design constraints.
To facilitate the distribution of the cooling air substantially completely over the convex and concave surfaces of the blade airfoil or platform, as shown in FIG. 10, the upstream end of each cooling hole 22 has a generally cylindrical, inlet portion 26 that extends from a location 28 inside of a wall of the component 20. At the location 28, the cooling hole 22 then flares or diverges to provide a discharge portion 30 that terminates on an exterior surface 32 of the component 20 to be cooled by the air flow. The shape of the discharge end functions as a diffuser to reduce the velocity of the cooling airstreams being discharged from the cooling holes 22. The lower velocity cooling airstreams are more inclined to cling to the surface 32 for improved cooling. High quality cooling holes 22 with diffusers 30 provide superior performance but are costly and difficult to manufacture.
One known method of forming the cooling holes 22 uses an electrodischarge machining (“EDM”) process, in which a first EDM tool is used to form the through-hole 26 and a second EDM tool is used to form the diffuser 30. For optimum performance, the diffuser must be very closely aligned with the through-hole; and that alignment is very difficult to maintain using multiple EDM tools.
Another known EDM process uses a single, comb-like tool that has a plurality of parallel EDM electrodes connected together at a common end to form a tool base. The parallel electrodes are spaced to correspond to the desired centerline spacing of adjacent cooling holes 22. The electrodes are shaped to correspond to the desired shape of the cooling hole; and therefore, a single electrode forms both the diffuser 30 and the through-hole 26. While this process often provides a consistent alignment of the diffuser cavity with the through-hole, the process is limited in that the comb-like tool is very fragile. The parallel electrodes are typically copper and are easily bent, and machining a turbine blade with a misaligned electrode may destroy the blade. Further, the comb-like tool often has to be replaced after only one machining cycle.
A further known process for forming the cooling holes 22 is a two step process. With one step, a laser drilling machine is used to drill the through-hole 26 of the cooling holes; and in another step, an EDM process is used to create the diffuser 30. This two step process requires a laser drilling machining and an EDM machine. Thus, after drilling the hole on the laser drilling machine, the part must be moved to the EDM machine in order to form the diffuser at the discharge end of the hole. The laser drilling process has the advantages of being able to very accurately locate each hole and drill each hole. However, the process of removing, remounting and precisely realigning the part on the EDM machine is very time consuming, and introduces opportunities for the stacking of errors. Further, laser drilling requires additional process steps of inserting material inside the part to absorb the laser beam when it breaks through to the part interior; and thereafter removing that material. In addition, a laser drilling machine is a very expensive piece of equipment; and the total time to drill and form all of the cooling holes in a part is substantial.
Another two step process is known in which a first EDM machine is used to drill the through-hole 26, and a second EDM machine uses a shaped tool to form the diffuser 30. While this process does not require an expensive laser drilling machine, it does have the disadvantages of requiring a shaped EDM tool and requiring the part be mounted and aligned on two different machines, which substantially increases the stacked error and the time required to process the part.
Thus, there is a continuing need for machinery and processes for forming complex cooling holes in gas turbine components that are faster, more flexible, more precise and less expensive than known machines and methods.